Certain properties of gypsum (calcium sulfate dehydrate) make it very popular for use in making industrial and building products; especially gypsum board. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed to useful shapes. The base material from which gypsum board is manufactured is the hemihydrate form of calcium sulfate (gypsum), commonly termed stucco, which is produced by the heat conversion of the dihydrate from which the water phase has been removed.
In the making of gypsum board, the gypsum slurry must flow onto a paper substrate. In a continuous process, the slurry/substrate combination is then sized by passing this combination between rollers. Simultaneous with this sizing step, a paper backing is positioned over the sized gypsum slurry. Accordingly, the gypsum slurry must possess sufficient fluidity so that a properly sized gypsum board can be made. Fluidity refers to the ability of the gypsum slurry to flow.
It is also important to the manufacture of gypsum board, that the gypsum slurry be capable of being foamed to a limited extent. Foamability refers to this ability to be foamed. When the gypsum slurry and paper substrate are passed though the sizing rollers, a certain amount of the gypsum slurry must back flow and accumulate in the rollers nip so that a steady flow of gypsum is delivered to the sizing rollers. Foamability is important to this ability of the gypsum slurry to back flow at the rollers nip. Forming plates may be used, eliminating the use of a master roll, but foam is important to control density of the finished product.
Because of the continuous nature of a gypsum board manufacturing process wherein the gypsum slurry flows onto a substrate which then passes through sizing rollers, the extent to which the gypsum slurry flows after it is sized is critical to maintaining the finished product dimensions of the gypsum board. The time at which the gypsum slurry ceases its flow is referred to as the pre-set time. Therefore, pre-set time is an important property of the gypsum slurry. The set time of the gypsum slurry is also an important property. The set time refers to the amount of time it takes the gypsum slurry to be dried, under heat, to the finished, solid gypsum board. As is well known in the art, in a continuous gypsum board manufacturing process, it is important that the gypsum slurry possess a consistent set time.
Gypsum board absorbs water, which reduces the strength of the wallboard. Prior art products, like ordinary gypsum board, gypsum tile, gypsum block, gypsum casts, and the like have relatively little resistance to water. When ordinary gypsum board, for example, is immersed in water, the board quickly absorbs a considerable amount of water, and loses a great deal of its strength. Actual tests have demonstrated that when a 2 inch by 4 inch cylinder of gypsum board core material was immersed in water at about 70° F., the cylinder showed a water absorption of 36% after immersion for 40 minutes.
Previous attempts to provide water-resistant properties to gypsum board include incorporation of asphalt, metallic soaps, resins, and wax additives into a aqueous gypsum slurry. The resulting materials were difficult to use and the core properties difficult to control. Polysiloxane-based systems have also been used in attempts to impart water-resistance to gypsum board. However, the polysiloxane-based systems are both expensive and difficult to use. A finished gypsum product has also been coated with water-resistant films or coatings. One specific example of a past attempt to provide a water-resistant gypsum product is the spraying of a molten paraffin, wax or asphalt into an aqueous gypsum slurry.
Another example of a prior art attempt to provide a water-resistant gypsum product is the addition of an emulsion of wax, such as paraffin wax, and asphalt, in the relative proportions of from about 1 part to about 10 parts of asphalt per part of wax to the aqueous gypsum slurry. Since the asphalt is a relatively poor solvent for paraffin wax and similar wax at ordinary temperatures, the solution formed at high temperatures tends on cooling to deposit microscopic wax crystals on the asphalt-wax surface.
Polyvinyl alcohol has been used in an attempt to provide a room temperature system for use in adding water-resistant properties to gypsum. However, the polyvinyl alcohol system tends to rapidly separate and thus typically requires continuous mixing prior to use. The inherent instability of the polyvinyl alcohol systems tends to produce stratification of the compounds in the formulation. Therefore, the polyvinyl alcohol systems tend to be compositionally inconsistent. In addition, because of destabilization into different phases, there is also the potential for bacterial growth.
Accordingly, there is a need for an additive which is useful in imparting water-resistance to gypsum products, and which is economical to apply. There is a need for a water-resistance additive which does not require the use of costly components such as polysiloxane. There is a need for a stable, water-resistance additive. There is a further need for a water-resistance additive which is stable at room temperate and which does not require heating prior to application to a gypsum solution. There is still a further need for a stable water-resistance additive which does not require continuous mixing or agitation to maintain its stability. There is yet a further need for a stable water-resistance additive which does not require the addition of a bactericide to control bacterial growth inherent in existing systems. Of course, such additives should perform these functions without affecting fluidity, foamability, pre-set time or set time.
Historically, products added to a gypsum slurry to impart a degree of water-resistance in the board manufacturing process have incorporated asphalt, molten wax, emulsified wax/asphalt, emulsified wax, and various silicone products. These prior art systems have all demonstrated shortfalls in any number of performance related areas. These shortfalls include, but are not limited to, inconsistent solids, instability of the emulsions, wide ranges in apparent viscosity, a caustic pH requiring hazardous labeling, health risks due to the evolution of hydrogen and hydrogen sulfide gases. An additive is needed that can address the aforementioned issues and impart water-resistance to a product.
It has been noted in earlier work that the incorporation of a generic starch species from corn, sago, wheat, rice, etc., with a complexing agent such as sodium borate in combination with other chemical compounds, specifically sodium lignosulfate, and C24 and greater polymerized alkyl phenol and various waxes forms a nearly stable wax emulsion suitable for incorporation into a gypsum slurry to impart water-resistance. While this system shows significant advantages over previously available wax emulsions it to suffers from a number of deficiencies, including: degradation of the pH due to bacteriological activity resulting from the decomposition of the sodium lignosulfate in long-term storage, viscosity changes as temperature and age occur manifesting itself as a slight separation at the water/wax interface, and less than predictable use rates at the mixer due to the changes occurring singularly and in combination.